Security/identity · 25 Dec 2005

The Patronus messaging system

As a Harry Potter fangirl of a certain (bossy) disposition, I got tagged with the nickname Hermione a couple of years ago. Hey, I can live with that, and I’ve even got the frizzy hair to match. Having now read book 6 twice through, I’m quite eager — desperate? — for book 7.

That’s all by way of explanation for why I was poking around J.K. Rowling’s site today. There, I found an interesting answer to a FAQ about the means of communication between members of the Order of the Phoenix. Passing messages safely, successfully, and confidentially is something we Muggles had to figure out properly in our physical world even before we had computers — but, naturellement, this topic put me in mind of secure end-to-end web service messaging.

Rowling’s answer harks back to information found way back in book 4, Goblet of Fire, so no spoiler warnings here. She explains:

Members of the Order use their Patronuses to communicate with each other. They are the only wizards who know how to use their spirit guardians in this way and they have been taught to do so by Dumbledore (he invented this method of communication). The Patronus is an immensely efficient messenger for several reasons: it is an anti-Dark Arts device, which makes it highly resilient to interference from Dark wizards; it is not hindered by physical barriers; each Patronus is unique and distinctive, so that there is never any doubt which Order member has sent it; nobody else can conjure another person’s Patronus, so there is no danger of false messages being passed between Order members; nothing conspicuous needs to be carried by the Order member to create a Patronus.

Let’s put this in slightly more technical terms and analogize the heck out of it, just for fun. Being naturally resistant to the Dark Arts is like using a modern ciphersuite for encryption. (I’m assuming that your Patronus is somehow instructed not to communicate with anyone besides the designated recipient so that you get confidentiality in that fashion, but I’m not sure she says that explicitly in the books.) Not being hindered by physical barriers is like allowing messages to pass end to end, traversing firewalls and different security domains with ease while retaining their integrity and confidentiality. Unique, un-forge-able Patronuses are akin to digital signing for data origin authentication, allowing detection of a false message inserted by a “wizard in the middle” (“Death Eater in the middle”?). As for needing nothing conspicuous to create your Patronus, if Rowling’s concern here was secrecy so that the sender could remain undetected as an OoP member, the closest analogue I can think of is the ability to obscure malicious traffic analysis. Alternatively, it could be like using a common off-the-shelf solution for doing your secure messaging (which is more about cost and convenience than security or secrecy — but since she referred to this as an “efficient” method, maybe that’s exactly what she meant).

All in all this works well enough that I wish I could use it in my occasional “Securing Your Web Services” talks! I should start asking for a show of hands on familiarity with the Potterverse; maybe I’d be pleasantly surprised.

p.s. I googled “Patronus web services” and “Patronus secure messaging” to see if anything came up, and got nothing directly related. I did get one interesting hit, though: a paper published by the American Bar Association called “The Patronus Technique: A Practical Proposal for Asbestos-Driven Bankruptcies”, about using special-purpose subsidiaries to distract litigious Dementors away from a larger corporate defendant. Whew. I thought I was being geeky about this.

p.p.s. Having used the phrase “we Muggles” above, I half-expect to get indignant mail from some people saying “speak for yourself”…

MORE: Kurt Cagle carries this line of thought even further in delightful fashion, even seeing a connection between Harry’s method of authentication for entering Sirius Black’s home and public key encryption. I’m gonna have to study that passage again — it’s an analogy that’s too cool not to use in a tutorial of some kind. And M. David Peterson (being far too kind and generous to me, as always) decides that the Order of the Phoenix represents object-oriented programming and must be battled! That’s a tough bet to take… Given that today is my first blogiversary (first month’s worth of stuff here), I’m especially delighted that my most recent thoughts got batted around a bit. Thanks, guys, and thanks to you all out there for “listening”.